

illiams.] NILES, WACHSMUTH. 159 



►urlington limestone is divided into two distinct formations, which they 

 jailed the "Lower" and "Upper Burlington limestone." The two 

 sections can be distinguished from each other by their lithologic char- 

 acters alone, but the distinction chiefly depends upon the different kinds 

 of crinoids found in the two divisions. In the lower section its upper 

 strata become interstratified with beds of chert, and the uppermost 

 stratum of chert forms the division between the two sections. The 

 crinoids below this cherty bed are smaller, less coarse in their general 

 features, and the ridges, spines, etc., are never so prominent as in the 

 species of the upper division. The inference is, that circumstances 

 were not so favorable to the growth of these animals during the depo- 

 sition of the lower strata. There is a similar marked distinction between 

 the crinoids of the Upper Burlington and those of the Keokuk limestone, 

 the latter being still larger in size and more prominent in feature. A 

 band of chert is also found between the Upper Burlington and the 

 Keokuk, and it appears from these facts that there was something in 

 the presence of siliceous material in the depositing waters during the 

 formation of the upper beds of both the Lower and Upper Burlington 

 divisions which was unfavorable to the growth and life of the inhabit- 

 ing crinoids, for as the chert appears the crinoids seem to have declined, 

 and finally all species became extinct before the completion of the chert. 

 The same fact is found true of the Mollusca, most of the species of the 

 two divisions being distinct. Lists of some of the better-known spe- 

 cies of the crinoids are appended, arranged under the names of the 

 formations to which they are exclusively restricted. 



If the view of the authors were found to be correct in fact, it would 

 signify that the cherts had their origin during the original deposition 

 of the rock, but even were it a fact, it may be questioned whether the 

 difference in fauna was not purely local, conditioned upon changed 

 character of the bottom. 



The series of Eeports of the Geological Survey of Illinois, 1 by Mr. 

 A. H. Worthen as Director, began with vol. T., in 1866. 



In the classification of the rocks of Illinois, the New York nomen- 

 clature was used for the subdivisions of the Silurian and Devonian 



1 Geological Survey of Illinois, A. H. Worthen, Director. 



Vol. I. Geology. 1866. 



Vol.11. Paleontology, Descriptions of Vertebrates, by J. S. Newbury and A. H. Worthen. 1866. 

 Descriptions of Invertebrates, by F. B. Meek and A. H. Worthen. Description of Plants, by Leo 

 Lesquereux. 



Vol. III. Geology and Paleontology. 1868. Geology, by A. H. Worthen. Paleontology, by P. B. 

 Meek and A. H. Worthen. 



Vol. IV. Geology and Paleontology. 1870. Geology, by A. H. Worthen. Paleontology, Vertebrates, 

 by Newberry and Worthen. Plants, by Lesquereux. 



Vol. V. Geology and Paleontology. 1873. Geology, by Worthen and James Shaw. Paleontology, 

 by Meek and Worthen. 



Vol. VI. Geology and Paleontology. 1875. Geology, by Worthen, G. C. Broadhead.E. T. Cox. Pale- 

 ontology, by O. St. John, Worthen, and Meek. 



Vol. VII. Geology and Paleontology. 1883. Geology, by A. H. Worthen. Paleontology by A. H. 

 Worthen, Orestes St. John, and S. A. Miller, with an addenda by Charles Wachsmuth and W. H. 

 Bar r is. 



